Scroll down to browse a list of FTP accounts that have already been set up in the Account Management section

Scroll up and enter a log in name in the Login field and a password below (and type a second round of the password below that)

If you leave '/demo' in the directory text box then the user of this FTP account will only have access to the /demo directory and all folders underneath it. To give the user access to the entire website delete everything except the forward slash.